Method for the treatment of metal



United States patent Hans Burkhardt, Oberursel (Tauus), near Frankfurtam Main, Germany, assignor to Vereinlgte Deutsche MetallwerkeAktiengesellschaft, Frankfurt am Main, a corporation of GermanyApplication November 22, 1949, Serial No. 128,744

In Germany October 1, 1948 Public Law 619, August 23, 1954 Patentexpires October 1, 1968 7 Claims. (Cl. 204-140) This invention relatesto a method and a furnace for the treatment, i. e. the melting andrefinng, of light metal, more particularly of aluminum scrap andaluminum alloy scrap.

Scrap metals of the kind referred to mostly contain particles of heaviermetals, especially iron and Copper, in the form of screws, wires,rivets, chips and the like, which when getting into the light metal meltand remaining therein, will be dssolved by the same and reduce thequality of the light metal or render it useless. On the other hand, itwould require too much expenditure to remove these small parts of heavymetals mechanically before the melting.

According to the conventional method, the light metal was melted offfrom the scrap metal supplied into an inclned furnace, for collection inthe lower parts of the furnace. By means of a dam provided at themelting-off point the heavy metal parts were preveted from also movngtowards the lower part of the furnace. It will be understood that insuch a melting-off process the loss or waste of light metal due tooxidaton is very high, the more so as these scrap metals mostly consistof parts having a large surface, and the melting requires a relativelyhigh amount of energy.

As is well known, the light metal melts originating from such scrapmetal charges, in order to remove detrimental impu-ities, such asoxides, nitrides and gases, such as hydrogen or alloying Components tobe reduced or to be removed, such as, e. g. magnesium, mostly have to besubjected to a refining treatment which` so far in general consisted instirring the melt with salts or introducing chlorine into the melt. Bothtreatments, i. e., the melting and the refining process, are bound toincrease the loss due to oxidation, and the feeding of chlorine is more-50 over disagreeable and requires the use of a larger number of feedingtubes which have to be exchanged rather oftentimes. A furtherdisadvantage of the existing process consists in that the heavy metaldssolved in the melt, especially iron, while it can partly be separatedby allowing the aluminides to settle, is difficult to remove from themelt in the form of said deposits, so that the danger is present thatthe heavy metal is cast with the melt.

It is the object of the present invention to provide means by which saiddrawbacks of the existing processes can be avoided.

With this object in view according to the present invention a bath ofmolten salt is used which has a higher specific gravity than the lightmetal, but a lower specific gravity than the parts of heavy metalsadmixed to the light metal, so that the same sink into the salt meltimmediately after the melting of the light metal and are therebyprevented from forming an alloy with the light metal.

For the salt bath the halides, more particularly the chlorides, of thealkaline earth metals, such as barium chloride, strontium chloride orcalcium chloride mixed 2. with barium chloride in a ratio of 4:1 to 1:4proved particularly suitable. A preferred composition comprises 78percent of barium chloride with 22 percent of calcium chloride, of aspecific gravity of about 3.45 and a melting temperature of about 600 C.or a composition corresponding to one of the aforementioned proportions,in which the barium salt, the calcium salt or strontium salt is partlyor entirely replaced by the corresponding fiuoride salts. Thus, fornstance, instead of 78 percent of barium chloride 68 percent of bariumchloride and 10 percent of barium fluoride may be used.

By application of salt mixtures it is possible to control the specificgravity and the melting temperature of the salt melt as required. It isalso possible, therefore, by means of the method according to thepresent invention, to separate from the metal salt undesirable alloyingcomponents of the light metal, such as, heavy metal aluminides, moreparticularly iron aluminides, and others which have a higher specificgravity than aluminum and thereby fall through the salt melt disposedthereunder. In order to carry out such a process the melting temperatureis kept as closely as possible above the solidifying temperature, takingcare that the salt melt at this temperature is still in a very liquidstate. The easy fusibility can be improved by admixing furtherlow-melting salts, such as, e. g., sodium chloride, potassiurn chlorideor other alkaline halides or magnesium halide, taking care that thespecific gravity of the salt melt does not exceed that of the metalmelt. The method, therefore, is also very suitable for the melting oflight metal containing heavy metals in an alloyed form, such as, in theform of heavy metal alumnides rather than in a mechanically adrnixedform.

In the method according to the invention the melting of the light metalmay be ettected by direct heating in the conventional manner.

It has been found that it is particularly favorable, however, especiallyas regards a reduction of the loss by oxidation, to eiect the heating bymeans of the salt bath. In this case only the salt bath is heated whichtransmits its heat to the light metal charge from below. The salt bathis advantageously heated by alternating current, by means of electrodes.

This method of melting light metals by means of a heated salt melt of ahigher specific gravity proved to be very advantageous and aifordssavings of expenses and time in operation, so that it can be used withgood success also where a removal of heavy metals from the light metalmelts is not intended.

According to the invention the refining of the molten light metal iseffected by halides as Components of the salt melt, by passing directcurrent through the salt and metal melts. The light metal melt in thiscase forms the anode. Hence, at its boundary surface with the salt melthalogens, such as, e. g., in case of a contents of chlorides in the saltmelt, chlorine, will be developed according to the formula 2Cl +2 =2Clwhich during its reaction with the light metal owing to its uniform finedispersal will effectuate a particularly thorough refining, avoiding allthe disagreeable auxiliary phenomenona which had been involved in theconventional feeding of chlorine from outside.

A furnace for use in connection with the process according to theinvention has been shown in the accompanying drawing, in a sectionalView.

Refer-ring to the drawing, it will be seen that the outer brick lining 1contains a salt melt 2 which has a higher' specific gravity than analuminum melt 8 superposed thereto, and a lower specific gravity thanthe heavy metals which are to be removed from the light metal melt. The

furnace isheated by alternating current of a low voltage which is passedthrough the salt melt by means of iron electrodes 3. When the lightmetal, which is charged from above, is melting, 'after the salt alreadyhas been molten, the admixed heavy metal parts will sink into the saltbath on to a screening plate or trough 4 by which they can be drawn outof the furnace from time to time, by means of rings 5 fastened to thescreening trough. In order to rcfine the light metal melt, directcurrent is passed through it by means of an electrode 6 and a carbon rod'7, the light metal melt 8 serving as a positive electrode. The no'tenlight metal is removed by a tap hole 9. An opening 10 serves todischarge the salt melt from the fnrnace before cleaning the same.

Through an opening 11 in the brick lining the filled-in salt is causedto melt by means of a burner (not shown) as the furnace is put intooperation, since the low electric conductivity of the salts in anon-molten condition renders it dilicult to melt them by the passage ofelectric current. If desred, an auxiliary electrode may be used insteadof the burner, in a manner known per se.

As an example for carrying out a melting and rening process a furnacemay be assumed whose inner diameter is 500 mm. The height of the saltmelt is 300 mm., corresponding to a weight of the salt mixture of 200kg., that of the metal melt is about 550 mm. corresponding to a weightof 300 kg. The phase voltage of the alternating heating current appliedmay be 40 volts. In this case, about 120 kilowatt hours are required forthe melting. By feedng an amount of direct current of 3,300 ampere hoursat a voltage of 36 vo lts the melt is freed from gases and oxides andthe magnesium content is reduce d by about 0.5 percent to about 0.2percent.

While the invention has been described in detail with respect to a nowpreferred example and embodiment of the invention it will be understoodby those skilled in the art after understauding the invention, thatvarious changes and modifications may be made without departing from thespirit and scope of the nvention and it is intended, therefore, to coverall such changes and modifications in the appended claims.

Having thus described my invention, I claim as new and desire to secureby Letters Patent:

1. A method of melting aluminum base metal scrap containing foreignheavy metal parts to separate such heavy metal parts from such 'aluminumbase metal which comprises melting the aluminum base metal while thescrap is supported by a substantially quiescent fused salt melt whichhas a higher specific gravity than said aluminum base metal and a lowerspecific gravity than the foreign heavy metal parts contained in suchscrap and permtting the heavy metal parts freed by the melting of thealuminum base metal to descend through the fused salt melt upon whichthe scrap being melted is supported.

2. A method in accordance with claim 1, in which said salt meltconprises a mixture of halides including at least one alkaline earthmetal halide.

3. A method in accordance with claim 1, in which said salt meltcomprises a mixture of halides including at least one calcium halide andat least one barium halide.

4. A method in accordance with claim 1 in which the salt meltessentially consists of `a mixture of at least one calcium halide and atleast one barium halide, said halides being selected from the groupconsisting of calcum chlo'ide, calcium fiuoride, barium chloride andbarium fluoride.

5. A method of melting aluminum base metal scrap containing foreignheavy metal parts to separate such heavy metal parts from such aluminumbase metal and refining the melted aluminum base metal which comprisesdisposing the aluminum base metal scrap on a substantially quiescentfused salt melt which has a higher specific gravity than said aluminumbase metal and a lower specific gravity than the heavy foreign metalparts, supplying heat of fusion to the aluminum base metal only throughthe salt melt to melt such aluminum base metal, and permitting the heavymetal parts freed by the melting of the aluminum base metal to descendthrough the fused salt melt upon which the scrap being melted issupported.

6. A method of melting *aluminum base metal scrap containing foreignheavy metal parts to separate such heavy metal parts from such aluminumbase metal which comprses melting the aluminum base metal while thes:rap is supported by a substantially quiescent fused salt meltcontaining halides which has a higher specific gravity than saidaluminum base metal and a lower specific gravity than the foreign heavymetal parts contained in such scrap, permittng the heavy metal partsfreed by the melting ot' the aluminum base metal to descend through thefused salt melt upon which the scrap being melted is supported andthereafter refining the resulting aluminum base metal melt while it issupported on said fused salt melt by passing a direct current throughthe salt melt with aluminum base metal melt as the anode.

7. A method in accordance with claim 6 in which said aluminum base metalis an aluminum base alloy containing magnesium.

References Cited in the file of this patent UNITED STATES PATENTS'800,934 Chance Oct. 3, 1905 850,726 Bills Apr. 16, 1907 1,513,875 WilkeNov. 4, 1924 1,534,318 Hoopes et al Apr. 21, 1925 1,564,139 SaklatwallaDec. 1, 1925 1,576,080 Beck Mar. 9, 1926 1,940, 6l8 Barstow et al Dec.19, 1933 1,940,619 Barstow et al Dec. 19, 1933 2,130,801 Hulse Sept. 20,1938 2,264,740 Brown Dec. 2, 1941 2,355,l30 Yerkes Aug. 8, 19442,426,389 Chew Aug. 26, 1947 2,620,269 Haney et al. Dec. 2, 1952 FOREIGNPATENTS 404563 Great Britain Jan. 18, 1934 489,700 Great Britain Aug. 2,1938 i 557,262 Great Britain Nov. 12, 1943

1. A METHOD OF MELTING ALUMINUM BASE METAL SCRAP CONTAINING FOREIGNHEAVY METAL PARTS TO SEPARATE SUCH HEAVY METAL PARTS FROM SUCH ALUMINUMBASE METAL WHICH COMPRISES MELTING THE ALUMINUM BASE METAL WHILE THESCRAP IS SUPPORTED BY A SUBSTANTIALLY QUIESCENT FUSED SALT MELT WHICHHAS A HIGHER SEPCIFIC GRAVITY THAN SAID ALUMINUM BASE METAL AND A LOWERSPECIFIC GRAVITY THAN THE FOREIGN HEAVY METAL PARTS CONTAINED IN SUCHSCRAP AND PERMITTING THE HEAVY METAL PARTS FREED BY THE MELTING OF THEALUMINUM BASE METAL TO DESCEND THROUGH THE FUSED SALT MELT UPON WHICHTHE SCRAP BEING MELTED IS SUPPORTED.